In recent years, much attention has been focused on bioremediation as a technology for the purification of contaminated soils. In bioremediation, contaminant-decomposing microorganisms (target microorganisms) that occur naturally are allowed to grow and the grown microorganisms are applied to the soil. Though soils can be purified by this method, many contaminant-decomposing microorganisms can remain in the soil, which is not desirable in terms of the balance of the soil ecosystem. Thus, as methods for removing contaminant-decomposing microorganisms that remain after the treatment, there are known methods such as gene recombinant technology that employs a suicide system (suicide gene) (Manual for Introduction of Recombinants into the Ecosystem, 1993, Ministry of Agriculture, Forestry, and Fisheries, P36-47), chemical and physical sterilization methods (Dictionary of Plant Physiology, edited by The Phytopathological society of Japan, Yokendo, 1995, pp. 701-719), and methods of imparting auxotrophic properties to target organisms.
However, each of these methods has its own problems. For example, since the method of using suicide system is based on recombinant technology it is not suitable for use in the field (an open system), and it usefulness is uncertain. The chemical and physical sterilization methods have drawbacks that all microorganisms including the soil microorganisms become the target of sterilization, and therefore may disturb the ecosystem. And, in the use of auxotrophic microorganisms, there is a risk that auxotrophy may disappear due to reverse mutation.